v CRUSTACEA ONTOGENY 399 



of the mesoderm ; the visceral layer never becomes segmented, and differentiates 

 quite apart from the parietal layer. (In the anal segment on each side there lie 2 

 large cells, which, however, do not divide and do nothing to increase the budding 

 zone by contributing cells.) 



The cell material of the mesoderm segments, which are successively formed at 

 the anterior end of the budding zone, begins to group itself into three divisions, and 

 this group is the more distinct the further the segment is removed from the budding 

 zone, i.e. the older it is. The dorsal division yields the rudiment of the cardial 

 chamber of the segment and that part of the dorsal longitudinal musculature which 

 also belongs to it ; the lateral division yields the musculature of the limbs, the ventral 

 the segmental division of the ventral musculature as well as the neurilemma of the 

 ganglia. The limbs begin to form as outgrowths and bulgings of the ectoderm, into 

 which cell-growths of the mesoderm penetrate. The ventral chord becomes differ- 

 entiated from before backward. In each segment in which a pair of limbs begins to 

 form an ectodermal thickening appears on each side of the ventral median line. The 

 two thickenings in a segment represent the rudiments of the double ganglion, which 

 at first are not united by a transverse commissure. These rudiments free themselves 

 from the ectoderm at a later stage. We thus find in an older Branchipus larva the 

 whole ventral chord from back to front in all stages of differentiation. Posteriorly, 

 where new pairs of ganglia are continuously formed with the new segments, these 

 are still ectodermal thickenings. In the formation of the heart (many-chambered 

 dorsal vessel) only one longitudinal row of perpendicularly arranged muscle cells on 

 each side takes part. The two rows grow together in such a way as to form a hollow 

 tube. The heart becomes differentiated from before backward. In many larval 

 stages a greater or smaller number of segmental cardial chambers are already developed 

 anteriorly and silre&dy pulsate, while posteriorly new cardial chambers are in process 

 of formation. 



The rudiments of the compound eye appear in the Metanauplius larva on each 

 side as a hypodermal thickening, which is then said to divide into two layers, a 

 superficial layer which yields the cornea and crystalline cone cells, and a deeper 

 layer which yields the retinulre with the rhabdoms. Another ectodermal thickening 

 connected with the first and belonging to the secondary brain chiefly yields the 

 material for the optic and the retinal ganglia. 



IV. Astacus fluviatilis (Figs. 270-280). 



The segmentation is superficial, and leads to the formation of a spherical blasto- 

 sphere (blastula), in which the central nutritive yolk, which is divided in radially 

 arranged yolk pyramids, is enveloped all round by the blastoderm as by an 

 epithelium. 



Stage A. At one point of the blastosphere the constitution of the blastoderm 

 undergoes a change. The cells at this part are longer and stand closer together. 

 This part corresponds with the future ventral side and is symmetrical ; it can be 

 denominated the ventral plate. This ventral plate consists of the following portions : 

 the 2 cephalic lobes, the 2 thoraco-abdominal rudiments, and farthest back, the 

 unpaired median endoderm disc (Fig. 271). The whole ventral plate is of one layer 

 except at one point between the endodermal disc and the thoraco-abdominal rudiments. 

 Large cells have sunk down inwards from the blastoderm as the primitive mesoderm 

 cells (B, M). "With the exception of the endoderm disc, which becomes invaginated 

 later, and the primitive mesoderm cells, the ventral plate and the whole remaining 

 blastoderm represents the ectoderm. The latter yields at a later stage the sides and 

 back of the thoracic shield, but in the early stages of development is nothing more 

 than a sac surrounding the yolk. 



